The development of portable devices such as personal computers and portable telephones has led to increasing demands for secondary batteries as the source of their power supply. In the secondary batteries for such purposes, a liquid electrolyte, such as organic solvent, is being widely used as a medium for causing ion movement. In a battery using the liquid electrolyte, there is a possibility that the electrolyte may become thermally unstable.
From the viewpoint of ensuring fundamental safety, development is underway for, e.g., an all-solid secondary battery in which solid electrolyte is used instead of liquid electrolyte, and in which all the other battery elements are configured of solids. In the all-solid secondary battery, the electrolyte is sintered ceramics, for example. Accordingly, the all-solid secondary battery has high thermal stability. In addition, the all-solid secondary battery is not prone to the problem of battery performance degradation due to corrosion, for example. Among others, an all-solid lithium secondary battery having electrodes in which lithium metal is used is believed to provide a secondary battery with which high energy density can be easily achieved.
In order to enhance the battery characteristics of a secondary battery, it is important to increase the potential difference between the material used in the positive electrode and the material used in the negative electrode, and to increase the capacity density of the materials used in the positive and negative electrodes. Particularly, with respect to the negative electrode material, it is known that use of lithium metal or lithium alloys greatly contribute to characteristics enhancement. However, when lithium metal is used in the negative electrode, dendrite (whisker-like crystal) may become deposited on the negative electrode as a result of charging and discharging. In a worst case, the dendrite may penetrate the separator and cause the problem of short-circuit between the positive and negative electrodes. Accordingly, due to safety concerns, lithium metal has not been used in the negative electrode. In the all-solid battery, in which the electrolyte portion is formed of a solid, deposits will not penetrate the solid electrolyte, and therefore it is expected that lithium metal can be safely used. However, lithium metal has the lowest potential and high reactivity. Accordingly, it has been difficult to obtain ceramics solid electrolyte in which lithium metal can be applied.
In recent years, it has been reported that Li7La3Zr2O12 (hereafter referred to as LLZ), which is a garnet-type Li-ion conductive oxide, does not readily react with lithium metal or lithium alloys used in negative electrode and therefore provides a stable solid electrolyte, and that the LLZ may possibly be utilized as a solid electrolyte for all-solid Li secondary battery (Angew. Chem. Int. Ed., 2007, 46, 7778-7781).